Twitter needs more people than WhatsApp because it does way more things than WhatsApp.
There's no json/XML 'feed' in WhatsApp. No one can just start following someone on WhatsApp to see all their messages in their feed. It doesn't work like that. I don't see every message my brother sends to his wife via WhatsApp even though I have separate chats with both hime and her. WhatsApp and Twitter are entirely different things. Twitter is far more complex. Why am I having to explain this to you?
In Twitter you have a feed, and you choose who can post into that feed. WhatsApp is a messaging service with chats that you actively have to be in for you to see information. Musk has 114,400,000 followers on Twitter. You think 114,400,000 people would be in a chat group on WhatsApp to see what he messages? How about Barack Obama? He has 133,300,000 followers on Twitter. How would I see both Musk's and Obama's messages in one chat unless they were both in it? WhatsApp is not Twitter, and to compare their headcount is just wrong, sorry.
You're comparing a store that sells fruit & veg to a supermarket that sells fruit & veg, dairy, frozen food, toys, garden furniture, power tools...